1. Field of the Invention
The present invention relates to a light source apparatus, and more particularly to a wavelength conversion-type light source apparatus that produces a light beam in the ultraviolet region of the spectrum using a nonlinear optical crystal.
2. Description of the Related Art
In the manufacturing process of a photomask plate for semiconductor exposure apparatus, for which a fine processing is required, it becomes necessary to inspect microscopic defects that may be contained in the plate. Further, in the case of a reticle on which a fine exposure pattern is written, an inspection must be performed to check whether the actual pattern is written correctly and whether there are any defects on the pattern.
For such purposes, semiconductor defect inspection equipment wherein the object under inspection is illuminated with continuous or high repetition pulsed light and changes in light intensity due to scattering, etc., for comparison are detected, is used. Such semiconductor defect inspection equipment is available in a variety of types, but regardless of the type, the resolution generally increases as the wavelength of the light source is made shorter. Accordingly, the light source apparatus of the type that produces inspection light in the deep ultraviolet region of the spectrum by wavelength conversion using nonlinear optical crystals has come into wide use in recent years.
However, if highly coherent light is used for semiconductor inspection, an unevenness in the spatial distribution of the light intensity, called speckle, will occur. As a result, it is desirable, as pointed out, for example, in Japanese Unexamined Patent Publication No, 2006-269455, to use ultraviolet light whose coherence is not very high, and therefore to use a longitudinal multimode laser which operates in a plurality of longitudinal modes as the fundamental light source.
As one of light sources that satisfy such requirements, a light source that uses an argon-ion laser, which continuously oscillates at 515 nm or 488 nm, and produces second-harmonic light (257 or 244 nm) by wavelength conversion in an internal resonator is proposed.
A light source that produces a 198.5-nm continuous output by sum-frequency mixing between 244-nm light as the second harmonic of an argon-ion laser and 1064-nm light from an Nd:YAG laser or the like has been developed. However, the argon-ion laser not only has an extremely poor oscillation efficiency and requires large power to operate, but also has a major problem in terms of maintenance since it requires frequent replacement of the gas laser tube.
In view of the above, vigorous research and development has been conducted in recent years to develop deep ultraviolet light sources of all-solid construction smaller in size and easier to maintain, examples of which include light sources that produce the fourth harmonic (266 nm) or the fifth harmonic (213 nm) of an Nd:YAG laser or Nd:YVO4 laser (1064 nm) and those that emit 193.4-nm light by generating the eighth harmonic of an erbium-doped fiber laser that emits 1540-nm light.
To produce deep ultraviolet light from the infrared light of a longitudinal multimode Nd:YAG laser or fiber laser or the like, at least two stages of wavelength conversion become necessary. It is therefore difficult, in principle, to place a nonlinear optical crystal for ultraviolet generation inside a laser resonator or an external resonator. To achieve a light source having a practical output, an infrared laser having a high peak intensity, for example, a high-output laser light source that produces highly repetitive pulses by mode-locking, is suitable for application.
In the case of deep ultraviolet light sources of 266 nm or shorter wavelengths, generally the efficiency of conversion from the input light to the output light decreases over time due to the optical damage or deterioration of the nonlinear optical crystal, regardless of the type of output light generation. It is therefore necessary to stabilize the output by increasing the input light power so as to compensate for the decrease. Usually, stabilization control is performed using an APC (Automatic Power Control) circuit by separating a portion of the output light and measuring its intensity and by increasing or reducing the output of the pump laser to compensate for the decrease or increase in the intensity of the output light.
A light source for semiconductor inspection requires not only long-term stability but also short-term output stability on the order of kilohertz in the frequency band, i.e., the suppression of high-frequency optical noise. Optical noise in wavelength-converted light has been a major problem to be solved in the development of a small-output green or blue light source in which a wavelength conversion crystal is placed inside the resonator of an internal resonator-type laser light source.
As a method for suppressing the noise, measuring a portion separated from the output and to feed the measurement back to the current to be supplied to the semiconductor pump laser has been proposed, for example, in Japanese Unexamined Patent Publication No. 9-232665, Japanese Unexamined Patent Publication No. 10-70333, etc.,. Since this method is directed to an electrical control, it is also possible to compensate for high-frequency optical noise. Various means are also known, such as the installation of a wave plate, the application of phase compensation, etc., which compensate for the mode competition within the resonator that causes optical noise peculiar to the internal resonator type.
For an ultraviolet light source used in semiconductor inspection equipment that detects defects by capturing subtle differences in the changing intensity of the laser light illuminating the object under inspection, it is important that short-term and long-term optical output fluctuations be small. For a similar reason, uniformity in the spatial distribution of the light intensity over the illuminated surface is also important.
The conversion efficiency of a wavelength conversion-type light source using a nonlinear optical crystal depends on the square of the intensity of input light (in the case of harmonic generation) or on the product of the intensities of two input lights (in the case of sum-frequency generation). As a result, if there is an output variation of 1% in the fundamental light, for example, it will result in an output variation of 4% in the case of fourth-harmonic generation which requires two stages of wavelength conversion.
If the output is to be stabilized by controlling the driving current of the semiconductor pump laser, as in the case of a conventional small-sized visible light source that performs wavelength conversion within a resonator, the amount of light absorbed into the laser medium changes, and the profile and the spreading angle of the output light change because of the change in the distribution of the refractive index of the laser medium, which is referred to as the thermal lens effect. As a result, the efficiency of wavelength conversion into the ultraviolet region drops, and the profile and the spreading angle of the resulting ultraviolet light change, rendering the method unsuitable for the purpose.
Further, in the case of a light source apparatus that employs as the fundamental light source a mode-locked laser using a semiconductor saturable mirror, which is currently the predominant type, if the pump light power is reduced, mode-locking cannot be maintained; therefore, in this case also, the method of controlling the output by adjusting the current of the semiconductor pump laser is not practicable.
If ultraviolet light is produced by nonlinear wavelength conversion using a longitudinal multimode light source that does not easily yield speckle as the fundamental light source, sum-frequency mixing occurs due to competition between longitudinal modes, and the problems that the fluctuation of the produced ultraviolet light output increases become pronounced. The aforementioned means, such as the installation of a wave plate, the application of phase compensation, etc., in the internal resonator-type wavelength conversion, are not effective in the case of a light source that produces ultraviolet light by higher-order harmonic generation which requires the provision of one or more nonlinear optical crystals outside the resonator.
As a means for eliminating or suppressing speckle, it is also effective to provide an optical means for degrading its spatial coherence. For example, Japanese Unexamined Patent Publication No. 2002-267825 discloses a rotating diffusion plate such as frosted glass or a diffractive lens element for use as such an optical means. However, if such a modulation element is used, the light output passing through the element fluctuates, if only slightly, with a period equal to the modulation, and this fluctuation can cause optical noise. Even when the amount of fluctuation is as small as about 2%, for example, it presents a non-negligible problem in the case of a light source for semiconductor inspection.
Japanese Unexamined Patent Publication No. 2000-164950 discloses an apparatus in which an attenuator constructed by combining a wave plate with a polarizer is placed between a laser light source and a nonlinear optical crystal. However, in this apparatus, the fluctuation of light that can be suppressed by rotating the wave plate is of one second order at most, and it cannot address kilohertz order optical noise.
Providing an apparatus called a noise eater for the elimination of conventional optical noise in laser light is also known. In this apparatus, an attenuator is constructed by combining an electro-optical crystal or the like with a polarizer, and feedback control is performed so as to stabilize the output of the light transmitted through the polarizer.
In such an apparatus, the optical transmitting power is basically fixed to a level not higher than the minimum value of the fluctuating output, and the optical loss of the polarizer is rather large; as a result, the laser light output drops to about 85% at the maximum. Furthermore, in the wavelength conversion, if the output of the pump light source is stabilized using a noise eater, it is not possible to eliminate optical noise that occurs during the wavelength conversion process such as the generation of sum-frequency due to longitudinal mode competition, nor is it possible to eliminate optical noise that arises from the means for degrading the spatial coherence affecting the wavelength-converted light.